1. Field of the Invention
The present invention relates generally to fuel systems on engines and, more specifically, to a circuit for limiting pull-in coil current in a solenoid such as a fuel shut-off solenoid having a coil that is temporarily activated on start-up of the engine.
2. Related Art
Fuel shut-off solenoids often are used on diesel engines to interrupt fuel flow from the injection fuel pump when the ignition is switched off. On start-up of the engine, a solenoid pull-in coil must be temporarily activated to turn on the solenoid. The pull-in coil can draw up to approximately fifty amps. Once the solenoid is activated, a hold-in coil which has a much lower current draw than the pull-in coil maintains the solenoid in the on condition, and the pull-in coil is turned off to avoid overheating. A typical starting circuit has the pull-in coil connected to the start terminal
on the ignition switch. If the key is held at the start position for an extended period of time or if the key sticks in the start position, the solenoid can overheat and burn out. Some starting circuits use an electronic timer to pulse the pull-in coil for a short time, but these circuits are more complex and expensive. A thermistor can be used in the pull-in coil circuit to decrease pull-in current as the thermistor heats, but some current continues to flow through the pull-in coil circuit and the thermistor remains hot after the solenoid is activated. If power to the fuel solenoid is cut off for any reason while the thermistor is hot, the solenoid cannot be reengaged to restart the engine until the thermistor cools. As a result, unwanted delays in engine operation and restarting can occur under certain conditions.
Another type of circuit, such as shown in U.S. Pat. No. 5,379,733 which is of common ownership with the present application, utilizes a relay responsive to an increase in the engine oil pressure to cut off pull-in coil current. Since oil pressure varies considerably under differing conditions, providing a consistent pull-in coil current pulse can be a problem.